High-set separated overfire air system for pulverized coal fired boilers

ABSTRACT

A furnace ( 10 ) includes a series of lower compartments ( 18 ) extending in a vertical arrangement, with at least one of the lower compartments ( 18 ) having one or more fuel nozzles ( 20 ) which tangentially fires fuel into the combustion chamber at an offset from a diagonal (DD) passing through opposed corners of the combustion chamber. At least one of the lower compartments ( 18 ) has one or more air nozzles ( 22 ) which tangentially introduces air into the combustion chamber along an offset direction which is offset from the diagonal (DD) to the same side as the fuel firing offset direction. The tangentially fired air and the offset fired fuel create a swirling fireball (RB) in the combustion chamber. At least one overfire compartment ( 24 ) has at least one air nozzle ( 40 ) for injecting air in opposition to the swirling fireball (RB), along an opposition offset direction which is offset to the other side of the diagonal (DD), in a manner such that the injected air promotes the evolution of the swirling fireball (RB) into an upward flow.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to a fossil fuel-fired furnace and a method of operating a fossil fuel-fired furnace. More particularly, the present invention relates to a pulverized coal-fired furnace and a method of operating a pulverized coal-fired furnace so as to control the flow of combustion products therein.

[0002] Pulverized solid fuel has been successfully burned in suspension in furnaces by tangential firing methods for a long time. The tangential firing technique involves introducing the pulverized solid fuel and air into a furnace from the four corners thereof so that the pulverized solid fuel and air are directed tangent to an imaginary circle in the center of the furnace. This type of firing has many advantages, among them being good mixing of the pulverized solid fuel and the air, stable flame conditions, and long residence time of the combustion gases in the furnaces.

[0003] It is known that a staged combustion approach can improve the reduction of NO_(x) in a fossil fuel-fired furnace such as, for example, a furnace in which pulverized coal is fired. Such a staged combustion approach may include reducing the quantity of air introduced into a main burner region of the furnace, which is a region in which the fuel such as the pulverized coal is injected, and instead introducing greater quantities, of air above the main burner zone.

[0004] Over the years, there have been different approaches pursued in the prior art insofar as concerns addressing the need to limit emissions of the NO_(x) that is created as a consequence of the combustion of fossil fuels in furnaces. The focus of one such approach has been on developing so-called low NO_(x) firing systems suitable for employment in fossil fuel-fired furnaces. U.S. Pat. No. 5,020,454 entitled “Clustered Concentric Tangential Firing System”, which issued on Jun. 4, 1991 and which is assigned to the same assignee as the present patent application discloses an example of one such low NO_(x) firing system. In accordance with the teachings of U.S. Pat. No. 5,020,454, a clustered concentric tangential firing system is provided that includes a windbox, a first cluster of fuel nozzles mounted in the windbox and operative for injecting clustered fuel into the furnace so as to create a first fuel-rich zone therewithin, a second cluster of fuel nozzles mounted in the windbox and operative for injecting clustered fuel into the furnace so as to create a second fuel-rich zone therewithin, an offset air nozzle mounted in the windbox and operative for injecting offset air into the furnace such that the offset air is directed away from the clustered fuel injected into the furnace and towards the walls of the furnace, a close-coupled overfire air nozzle mounted in the windbox and operative for injecting close-coupled overfire air into the furnace, and a separated overfire air nozzle mounted in the windbox and operative for injecting separated overfire air into the furnace.

[0005] Another example of such a low NO_(x) firing system is that which forms the subject matter of U.S. Pat. No. 5,315,939 entitled “Integrated Low NO_(x) Tangential Firing System”, which issued on May 31, 1994 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,315,939, an integrated low NO_(x) tangential firing system is provided that includes pulverized solid fuel supply means, flame attachment pulverized solid fuel nozzle tips, concentric firing nozzles, close-coupled overfire air, and multi-staged separate overfire air and when employed with a pulverized solid fuel-fired furnace is capable of limiting NO_(x) emissions therefrom to less than 0.15 lb./million BTU while yet maintaining carbon-in-flyash to less than 5% and CO emissions to less than 50 ppm.

[0006] Both of the tangentially fired furnaces disclosed in the two afore-mentioned references capitalize on the knowledge that the formation of NO_(x) in a tangentially fired furnace can frequently be minimized by judicious control of the air introduced above the fuel-rich main burner zone—i.e. the introduction of so-called overfire air. Judicious control of the overfire air in such circumstances is characterized by the introduction of the overfire air in a manner which supports the formation of the swirling fireball in the furnace while also supporting the sub-stoichiometric conditions in the main burner zone. With regard to supporting the sub-stoichiometric conditions in the main burner zone, it can be appreciated that any increase in the residence time of the fuel in the sub-stoichiometric (fuel rich) main burner zone will further promote the reduction of NO_(x).

[0007] Notwithstanding the fact that over the years there have been different approaches disclosed in the prior art targeted at the reduction of emissions of the NO_(x) that is created as a consequence of the combustion of fossil fuels in furnaces, a need still exists in the prior art to improve upon what has been accomplished in the pursuance of these different approaches. For example, the need still exists for an approach which would permit the introduction of overfire air in a manner which promotes a longer residence time of fuel in the sub-stoichiometric conditions of the main burner zone of a tangentially fired furnace while at the same time minimizing the energy required to accomplish an introduction of overfire air in this manner.

SUMMARY OF THE INVENTION

[0008] Briefly stated, the invention in a preferred form is a pulverized coal-firing furnace which includes a series of lower compartments extending in a vertical arrangement, with at least one of the lower compartments having one or more fuel nozzles which tangentially fires fuel into the combustion chamber at an offset from a diagonal passing through opposed corners of the combustion chamber. The top most lower compartment having at least one fuel nozzle has a centerline at a height H_(tot) from the boiler nose. At least one of the lower compartments has one or more air nozzles which tangentially introduces air into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction. The tangentially fired air and the offset fired fuel create a swirling fireball in the combustion chamber. At least one overfire compartment located at a highset overfire position has at least one air nozzle for injecting air in opposition to the swirling fireball, along an opposition offset direction which is offset to the other side of the diagonal, in a manner such that the injected air promotes the evolution of the swirling fireball into an upward flow. The centerline of the highset overfire position is at a height H_(sofa) from the centerline of the topmost fuel injection compartment, where 0.5≦(H_(sofa)/H_(tot))≦0.9.

[0009] In a first alternative, the pulverized coal-firing furnace may include a series of lower compartments extending in a vertical arrangement, with at least one of the lower compartments having one or more fuel nozzles which tangentially fires fuel into the combustion chamber at an offset from a diagonal passing through opposed corners of the combustion chamber. At least one of the lower compartments has one or more air nozzles which tangentially introduces air into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction. The tangentially fired air and the offset fired fuel create a swirling fireball in the combustion chamber. At least one overfire compartment is located at a vertical distance from the topmost lower compartment which is greater than the vertical distance between any given one of the lower compartments and an adjacent lower compartment. The overfire compartment includes at least one high velocity air nozzle and at least one low velocity air nozzle for injecting air in opposition to the swirling fireball, along an opposition offset direction which is offset to the other side of the diagonal, in a manner such that the injected air promotes the evolution of the swirling fireball into an upward flow. High velocity air jets from the high velocity air nozzle penetrate to the axis, preventing a fuel rich core at center of the boiler, and low velocity air jets from the low velocity air nozzle sweep the furnace walls, preventing fuel rich pockets proximate to the furnace walls.

[0010] The free area of the low velocity air nozzle is substantially equal to three times the free area of the high velocity air nozzle. The high velocity air nozzle me be disposed in a first overfire compartment and the low velocity air nozzle may be disposed in a second overfire compartment, where the air flow of the second overfire compartment is substantially equal to the air flow of the first overfire compartment. Accordingly, the walls of the second overfire compartment and a damper disposed therein define a restricted passage for creating a pressure drop in the flow passage.

[0011] In a second alternative, the pulverized coal-firing furnace may include a series of lower compartments extending in a vertical arrangement and including an upper first lower compartment, a lower second lower compartment, and a lowest third lower compartment. The first and third lower compartments each have one or more fuel nozzles which tangentially fires fuel into the combustion chamber at an offset from a diagonal passing through opposed corners of the combustion chamber. The second lower compartment has upper, intermediate, and lower sub-compartments and a plurality of air nozzles which tangentially introduces air into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction. One of the air nozzles is associated with each of the sub-compartments. The tangentially fired air and the offset fired fuel create a swirling fireball in the combustion chamber. At least one overfire compartment, located at a vertical distance from the topmost lower compartment which is greater than the vertical distance between any given one of the lower compartments and an adjacent lower compartment, has at least one air nozzle for injecting air in opposition to the swirling fireball, along an opposition offset direction which is offset to the other side of the diagonal, in a manner such that the injected air promotes the evolution of the swirling fireball into an upward flow.

[0012] The second lower compartment includes a single tilt control and each of the sub-compartments includes a yaw control. One or more of the sub-compartments may have an offset from the diagonal which is different from the offset from the diagonal of the other sub-compartments.

[0013] In a third alternative, the pulverized coal-firing furnace may include a series of lower compartments extending in a vertical arrangement, with at least one of the lower compartments having one or more fuel nozzles which tangentially fires fuel into the combustion chamber at an offset from a diagonal passing through opposed corners of the combustion chamber. At least one of the lower compartments has one or more air nozzles which tangentially introduces air into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction. The tangentially fired air and the offset fired fuel create fireball rotating in a swirl direction in the combustion chamber. At least one overfire compartment, located at a vertical distance from the topmost lower compartment which is greater than the vertical distance between any given one of the lower compartments and an adjacent lower compartment, has at least one air nozzle for injecting air in opposition to the rotating fireball, along an opposition offset direction which is offset to the other side of the diagonal, in a manner such that the injected air promotes the evolution of the rotating fireball into an upward flow. The fuel nozzle of the lowermost lower compartment injects fuel into the combustion chamber fires fuel vertically downward and in the opposition offset direction.

[0014] It is an object of the invention to provide furnace having a separated overfire air system set at an elevation which maximizes the substoichiometric residence time of the combustion gases in the boiler, thereby lowering NOx emissions.

[0015] It is also an object of the invention to provide a furnace having a separated overfire air system which may vary the velocity of the coal injected into the furnace to reduce NOx emissions.

[0016] It is further an object of the invention to provide a furnace using sub-compartmentalization of the auxiliary air compartments to provide greater control of the near field stoichiometry, thereby lowering NOx emissions.

[0017] It is still further an object of the invention to provide a furnace which reduces NOx emissions by controlling the trajectory of the coal injected by the lower coal elevation.

[0018] Other objects and advantages of the invention will become apparent from the drawings and specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which:

[0020]FIG. 1 is a schematic perspective view of a fossil fuel-fired furnace equipped with a tangential firing system and having a first embodiment of the corner windbox air compartment of the present invention;

[0021]FIG. 2 is an enlarged perspective view of the first corner windbox of the furnace shown in FIG. 1;

[0022]FIG. 3 is an enlarged perspective view of one variation of the corner windboxes of the furnace shown in FIG. 1;

[0023]FIG. 4 is simplified schematic view of the furnace of FIG. 1 showing the relative position of the high set separated overfire air compartment;

[0024]FIG. 5 is an enlarged, simplified cross section view of large overfire air nozzle of FIG. 3;

[0025]FIG. 6 is an enlarged perspective view of lower compartment 18C of FIG. 2;

[0026]FIG. 7 is an enlarged side elevational view of a variation of the bottom compartment of FIG. 1;

[0027]FIG. 8 is a perspective view of the nozzle of the bottom compartment of FIG. 7; and

[0028]FIG. 9 is a graph illustrating the additional NO_(x) emissions reduction as a function of the orientation of the nozzle of the bottom coal injection compartment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] With reference to the drawings wherein like numerals represent like parts throughout the several figures, a fossil fuel-fired furnace 10 has a plurality of walls 11 forming a burner region 12 in which a combustion process is sustained by a tangential firing system 14.

[0030] The tangential firing system 14 is preferably of the type commonly denominated as a concentric tangential firing system. The concentric tangential firing system 14 is operable in a burner region 12 of a fossil fuel-fired furnace 10, which may be a pulverized coal-fired furnace. The burner region 12 defines a longitudinal axis BL extending vertically through the center of the burner region 12.

[0031] The burner region 12 has four corners each substantially equidistant from adjacent corners such a combustion chamber thus formed by the burner region has a parallelepiped shape which may be, for example, a rectangular or square shape. In the four corners of the combustion chamber are arranged a first windbox 16A, a second windbox 16B, a third windbox 16C, and a fourth windbox 16D. The first windbox 16A is generally circumferentially intermediately disposed between the second windbox 16B and the fourth windbox 16D as viewed in a circumferential direction relative to the burner region longitudinal axis BL such that the first windbox 16A is at a generally equal circumferential spacing from each respective one of the second windbox 16B and the fourth windbox 16D. The third windbox 16C is generally circumferentially intermediately disposed between the second windbox 16B and the fourth windbox 16D on the respective other side of these windboxes as viewed in the circumferential direction such that the third windbox 16C is at a generally equal circumferential spacing from each respective one of the second windbox 16B and the fourth windbox 16D.

[0032] The first windbox 16A and the third windbox 16C define a first pair of juxtaposed windboxes in juxtaposed relation to one another (i.e., the pair of windboxes are disposed on a diagonal DD passing through the longitudinal axis BL). The second windbox 16B and the fourth windbox 16D define a second pair of juxtaposed windboxes in juxtaposed relation to one another.

[0033] The windboxes 16A-16D each comprise a plurality of compartments which will now be described in greater detail with respect to the first windbox 16A which is hereby designated for this descriptive purpose as a representative windbox, it being understood that the other windboxes 16B-16D are identical in configuration and operation to this representative windbox. The first windbox 16A includes a series of lower compartments 18 each for introducing therethrough fuel, air, or both fuel and air such that a combination of air and fuel is introduced into the combustion chamber via this series of lower compartments 18. It is to be understood, however, that one or more of the windboxes 16A-16D can alternatively be configured such that its series of lower compartments 18 only introduce a selected one of fuel or air into the burner region 12, as desired. The lower series of compartments 18 extend into the bottom half BH of the furnace in a vertical arrangement with the series of lower compartments 18 being successively located one below another in an extent from a topmost one of the lower compartments, designated the top lower compartment 18T to a bottommost one of the lower compartments.

[0034] The first windbox 16A further includes a plurality of fuel nozzles each suitably mounted in selected ones of the lower compartments 18 for tangentially firing fuel into the combustion chamber. As seen in FIG. 2, two of the fuel nozzles 20 are representatively shown in mounted disposition in representative lower compartments 18 of the type provided with a fuel nozzle, these representative compartments being hereinafter designated as lower compartments 18A and 18B. The fuel nozzles 20 disposed in the lower compartments 18A, 18B fire fuel and primary air in a direction tangential to a fireball RB that rotates or swirls generally about the longitudinal axis BL of the burner region 12 while flowing upwardly therein. The tangential fuel firing direction (or offset fuel firing direction) is at an angle from the diagonal DD.

[0035] The first windbox 16A further includes a plurality of air nozzles 22 for introducing secondary air from other ones of the lower compartments 18 into the combustion chamber tangential to the rotating fireball RB. The air nozzle 22 introduces air along a air offset direction which is offset from the diagonal DD in the same direction as the offset fuel firing direction. The offset fired fuel and air create and sustain the swirling or rotating fireball RB in the combustion chamber. Lower compartment 18C is one of the respective lower compartments dedicated to introducing secondary air into the furnace 10. The air collectively introduced via both the primary air nozzle portions of the fuel nozzles 20 and the secondary air nozzles 22 mounted in the lower compartments 18 is in an amount less than the amount required for complete combustion of the fuel fired into the burner region 12 such that the portion of the burner region 12 associated with the lower compartments 18 is characterized by a sub-stoichiometric combustion condition.

[0036] The furnace 10 additionally includes one or more overfire air compartments 24 which are disposed at a vertical distance from the top lower compartment 18T which is greater than the vertical distance between any given pair of adjacent lower compartments 18. The overfire air compartments 24 are operable to introduce separated overfire air (SOFA) into an upper region of the furnace 10 above the burner region 12 in opposition to the swirling fireball RB. That is, the overfire air is injected along an air offset direction which is offset to the opposite side of the diagonal DD compared to the offset fuel firing direction and the air offset direction of the air injected by the lower compartments. In prior furnaces having SOFA, a low set elevation of SOFA was initially utilized to reduce NOx emissions. In a subsequent advancement (as disclosed in U.S. Pat. No. 5,315,939), two elevations of SOFA were utilized with an upper overfire air compartment providing increased NOx reduction and a lower overfire air compartment providing acceptable levels of unburned carbon in the fly ash.

[0037] With reference to FIG. 4, the subject high-set separated overfire air system utilizes a single high-set SOFA 26 having an elevation specified according to the equation

0.5≦(H _(sofa) /H _(tot))≦0.9

[0038] where H_(sofa) is the height from the centerline 28 of the top coal injection compartment 18A to the centerline 30 of the high-set SOFA elevation and H_(tot) is the height from the centerline 28 at the top coal injection compartment 18A to the boiler nose 32 (where the cross-sectional area of the burner region 12 decreases by at least twenty percent). It should be appreciated that the subject invention utilizes only a high-set SOFA 26, there being no lower elevation of SOFA as was utilized in U.S. Pat. No. 5,315,939. The high-set SOFA may be provided by a single overfire air compartment 24 or multiple overfire air compartments 34, 36 (FIG. 3) which are vertically stacked. The high-set SOFA system 38 maximizes the substoichiometric residence time of the combustion gases in the boiler resulting in lower NOx emissions than the traditional low set SOFA systems.

[0039] To achieve high levels of carbon conversion, resulting in low CO emissions and low carbon in the fly ash levels, thorough mixing of the overfire air with the flue gas in the boiler is needed. The required SOFA mixing may be achieved in the high-set SOFA system 38 by introducing the air through nozzles 40 in the corners in a typical tangential fired arrangement, through nozzles (not shown) located on the wails of the boiler, or any combination of both corner and wall nozzles. As needed, the high-set SOFA system 38 may also incorporate the use of a boost fan to increase the velocity of the overfire air which may improve the mixing of the overfire air with the fine gas in the boiler.

[0040] Alternatively, the required SOFA mixing may be achieved with the variable velocity SOFA assembly 42 shown in FIGS. 3 and 5. Modeling of tangentially-fired boilers has shown a tradeoff between SOFA velocity and jet penetration and mixing. If the SOFA velocity is too low, the air will not penetrate to the center of the boiler resulting in a fuel rich core. If the SOFA velocity is sufficiently increased the jets will penetrate to the center of the boiler, but there may be some fuel rich pocket(s) remaining near the furnace walls 11. SOFA nozzle yaw may be used to put more air near the furnace walls 11, but increasing SOFA yaw also has the effect of increasing the furnace swirl which decreases SOFA jet penetration.

[0041] The variable velocity SOFA assembly 42 utilizes a combination of high velocity SOFA jets 44 and low velocity SOFA jets 46 to provide the additional flexibility needed to improve SOFA mixing in tangentially-fired boilers. The high velocity SOFA jets 44 penetrate to the center of the boiler while the low velocity jets 46 sweep the furnace walls 11 with less impact on the furnace swirl.

[0042] In a preferred embodiment, the variable velocity SOFA assembly 42 includes at least one overfire air compartment 36 having a single large SOFA nozzle 48, with a larger free area (approximately 3 times greater) than would typically be utilized, provides a low velocity SOFA jet 46. At least one overfire air compartment 34 having a conventional size SOFA nozzle 50 provides SOFA jets 44 having the maximum velocity allowed by the available fan. The variable velocity SOFA assembly 42 is designed such that the air flow through all of the compartments 34, 36 would be equal with the pressure drop through the large SOFA compartment 36 being taken across the restricted passage formed therein by the walls 51 of the compartment 36 and a damper 52. The additional SOFA free area allows for easy variation of the SOFA quantity for optimization during boiler tuning. Each of the SOFA compartments 34, 36 has independent yaw control.

[0043] An additional reduction in NOx emissions can be obtained by controlling the near field stoichiometry using windbox sub-compartmentalization. With reference to FIG. 6, an air injection compartment 18C located between a pair of coal injection compartments 18A, 18B may be divided into three smaller sub-compartments 54, 56, 58, each having its own air nozzle 60, 62, 64. Splitting the single air nozzle 22 into three individual air nozzles 60, 62, 64 increases the surface area of the resulting air jets, resulting in more rapid entrainment of flue gas which decreases the local oxygen concentration. A separate damper 66, 68, 70 may be provided for each sub-compartment 54, 56, 58, or alternatively the middle sub-compartment 56 may have one damper and the upper and lower sub-compartments 54, 58 may have a common damper, providing increased control over the near burner stoichiometry.

[0044] Preferably, the sub-compartments 54, 56, 58 will have a common tilt control 72, providing for the selective bias of auxiliary air toward or away from the adjacent coal nozzles 20, 20′ as required for windbox optimization. The sub-compartments 54, 56, 58 may be provided with separate yaw controls 74. Depending on the specific installation, one or more of the sub-compartment(s) may be offset while the remaining sub-compartment(s) are straight. For example, natural gas may be introduced into the furnace 10 as an auxiliary fuel through the upper and lower sub-compartments 54, 58 while air is injected through the middle sub-compartment 56. In this example, the nozzles 60, 64 of the upper and lower sub-compartments 54, 58 would be straight (injecting the auxiliary fuel along the same offset fuel firing direction as the fuel injected by the lower compartments) and the nozzle 62 of the middle sub-compartment 56 would inject the air along an air offset direction which is offset to the opposite side of the diagonal DD compared to the offset fuel firing direction.

[0045] Incorporating both straight and offset air nozzles provides maximum flexibility to provide air near the boiler walls 11 for protection and the ability to bias the air away from the coal nozzles 20, 20′ for maximum NOx reduction. Testing has shown NOx reductions of 0.01-0.02 lb/MMBtu when using windbox sub-compartmentalization as shown in FIG. 6.

[0046] An additional modest reduction in NOx emissions can be obtained by controlling the trajectory at which the lower elevation coal is injected into the boiler. A significant, repeatable NOx reduction was observed in testing when the nozzle 20′ of the bottom coal injection compartment 18B was oriented down and against the swirl direction.

[0047] With reference to FIGS. 7 and 8, a bottom coal injection compartment 18B has a nozzle 20′ with a 15 degree fixed offset along an offset fuel firing direction which is to the opposite side of the diagonal DD compared to the offset fuel firing direction for the rest of the fuel. This fixed offset nozzle has both tilt and yaw controls 76, 78 and can be rotated so that the coal is injected at ±15° tilt or at ±15° yaw, depending on the degree of nozzle rotation.

[0048] NOx emissions produced by burning either a sub-bituminous coal from the Powder River Basin (PRB) or a high volatile bituminous coal (HVB) responded to changes in the orientation of nozzle 20′ in substantially the same way and to substantially the same degree, as shown in FIG. 9. That is, the graph of the NOx emissions of the PRB coal has substantially the same form as the graph of the NOx emissions of the HVB coal. Tilting the nozzle 20′ of the bottom coal injection compartment 18B down toward the hopper and against the direction of furnace swirl causes the lower elevation of coal particles to follow a path more conducive to NOx reduction by controlling the stoichiometry of combustion and increasing the staged residence time.

[0049] While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation. 

1. A pulverized coal-firing furnace comprising: a combustion chamber having a boiler nose, a burner region disposed below the boiler nose, and four comers, each of the comers being substantially equidistant from each adjacent corner; a series of lower compartments for introducing therethrough one of air, fuel, or air and fuel into the combustion chamber, the series of lower compartments extending in a vertical arrangement from a topmost one of the lower compartments to a bottommost one of the lower compartments; at least one fuel nozzle for tangentially firing fuel from the series of lower compartments into the combustion chamber at an offset from a diagonal passing through one pair of opposed comers of the combustion chamber, the at least one fuel nozzle defining a topmost fuel injection compartment having a centerline at a height H_(tot) from the boiler nose; at least one air nozzle for tangentially introducing air from the lower compartments into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction, the air fired tangentially from the lower compartments being in an amount less than the amount required for complete combustion with the fuel such that the offset fired fuel and air create a swirling fireball in the combustion chamber; and a highset overfire position having a centerline and at least one overfire compartment having at least one air nozzle for injecting air from the at least one overfire compartment generally in opposition to the swirling fireball along an opposition offset direction which is offset to the other side of the diagonal in a manner such that the injected air promotes the evolution of the swirling fireball into an upward flow, the centerline of the highset overfire position being disposed at a height H_(sofa) from the centerline of the topmost fuel injection compartment, wherein 0.5≦(H_(sofa)/H_(tot))≦0.9.
 2. The furnace of claim 1 wherein there are no overfire positions disposed between the highset overfire position and the topmost fuel injection compartment.
 3. A pulverized coal-firing furnace comprising: a combustion chamber having a burner region, four corners, and four walls defining an axis extending vertically through a center of the burner region, each of the corners being substantially equidistant from each adjacent corner; a series of lower compartments for introducing therethrough one of air, fuel, or air and fuel into the combustion chamber, the series of lower compartments extending in a vertical arrangement from a topmost one of the lower compartments to a bottommost one of the lower compartments; at least one fuel nozzle for tangentially firing fuel from the series of lower compartments into the combustion chamber at an offset from a diagonal passing through one pair of opposed corners of the combustion chamber; at least one air nozzle for tangentially introducing air from the lower compartments into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction, the air fired tangentially from the lower compartments being in an amount less than the amount required for complete combustion with the fuel such that the offset fired fuel and air create a swirling fireball in the combustion chamber; at least one overfire compartment for injecting air into the combustion chamber, the at least one overfire compartment being disposed at a vertical distance from the topmost lower compartment which is greater than the vertical distance between any given one of the lower compartments and an adjacent lower compartment; and at least one high velocity air nozzle and at least one low velocity air nozzle for injecting air from the at least one overfire compartment generally in opposition to the swirling fireball along an opposition offset direction which is offset to the other side of the diagonal in a manner such that the injected air promotes the evolution of the swirling fireball into an upward flow, whereby high velocity air jets from the at least one high velocity air nozzle penetrate to the axis, preventing a fuel rich core at center of the boiler, and low velocity air jets from the at least one low velocity air nozzle sweep the furnace walls, preventing fuel rich pockets proximate to the furnace walls.
 4. The furnace of claim 3 wherein the at least one overfire compartment includes a first overfire compartment and a second overfire compartment, the at least one high velocity air nozzle being disposed in the first overfire compartment and the at least one low velocity air nozzle being disposed in the second overfire compartment.
 5. The furnace of claim 4 wherein the at least one high velocity air nozzle and the at least one low velocity air nozzle each have a free area, the free area of the at least one low velocity air nozzle being substantially equal to three times the free area of the at least one high velocity air nozzle.
 6. The furnace of claim 5 wherein each of the overfire compartments has an air flow, the air flow of the second overfire compartment being substantially equal to the air flow of the first overfire compartment.
 7. The furnace of claim 6 wherein the second overfire compartment includes a plurality of walls defining a flow passage and a damper disposed in the flow passage, the walls and damper defining a restricted passage for creating a pressure drop in the flow passage.
 8. The furnace of claim 4 wherein each of the overfire compartments has an independent yaw control.
 9. A pulverized coal-firing furnace comprising: a combustion chamber having a burner region, four corners, and four walls defining an axis extending vertically through a center of the burner region, each of the corners being substantially equidistant from each adjacent corner; a series of lower compartments for introducing therethrough one of air, fuel, or air and fuel into the combustion chamber, the series of lower compartments extending in a vertical arrangement and including an upper first lower compartment, a lower second lower compartment, and a lowest third lower compartment, at least one of the upper first the lower second, and the lowest third lower compartments having at least one fuel nozzle for tangentially firing fuel into the combustion chamber at an offset from a diagonal passing through one pair of opposed corners of the combustion chamber, at least one of the upper first, the lower second, and the lowest third lower compartments having a plurality of air nozzles for tangentially introducing air from the lower compartments into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction, the air fired tangentially from the lower compartments being in an amount less than the amount required for complete combustion with the fuel such that the offset fired fuel and air create a swirling fireball in the combustion chamber; at least one overfire compartment for injecting air into the combustion chamber, the at least one overfire compartment being disposed at a vertical distance from the topmost lower compartment which is greater than the vertical distance between any given one of the lower compartments and an adjacent lower compartment; and at least one air nozzle for injecting air from the at least one overfire compartment generally in opposition to the swirling fireball along an opposition offset direction which is offset to the other side of the diagonal in a manner such that the injected air promotes the evolution of the swirling fireball into an upward flow.
 10. The furnace of claim 9 wherein each of the upper first, the lower second, and the lowest third lower compartments includes a damper.
 11. The furnace of claim 9 wherein at least one of the upper first, the lower second, and the lowest third lower compartments has upper intermediate, and lower sub-compartments and, in the at least one of the upper first, the lower second, and the lowest third lower compartments having an upper, intermediate, and lower sub-compartment, the intermediate sub-compartment includes a damper and the upper and lower sub-compartments include a common damper.
 12. The furnace of claim 9 wherein the second lower compartment includes a single tilt control.
 13. The furnace of claim 11 wherein each of the sub-compartments includes a yaw control.
 14. The furnace of claim 11 wherein a one of the sub-compartments has an offset from the diagonal which is different from the offset from the diagonal of the other sub-compartments.
 15. The furnace of claim 11 wherein each of the sub-compartments has an offset from the diagonal which different from the offset from the diagonal of each other sub-compartment.
 16. The furnace of claim 11 wherein one or two of the sub-compartments include at least one fuel nozzle for tangentially firing fuel into the combustion chamber at an offset from the diagonal.
 17. A pulverized coal-firing furnace comprising: a combustion chamber having a burner region, four corners, and four walls defining an axis extending vertically through a center of the burner region, each of the corners being substantially equidistant from each adjacent corner; a series of lower compartments for introducing therethrough one of air, fuel, or air and fuel into the combustion chamber, the series of lower compartments extending in a vertical arrangement from a topmost one of the lower compartments to a bottommost one of the lower compartments; a plurality of fuel nozzles for firing fuel from the series of lower compartments into the combustion chamber, a lowest one of the lower compartments having at least one fuel nozzle defining a lowermost fuel injection compartment, each other of the lower compartments having at least one fuel nozzle defining an upper fuel injection compartment, the fuel nozzles of each upper fuel injection compartment tangentially firing fuel at an offset direction which is offset to one side of a diagonal passing through one pair of opposed corners of the combustion chamber; at least one air nozzle for tangentially introducing air from the lower compartments into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction, the air fired tangentially from the lower compartments being in an amount less than the amount required for complete combustion with the fuel such that the offset fired fuel and air create a fireball rotating in a swirl direction in the combustion chamber; at least one overfire compartment for injecting air into the combustion chamber, the at least one overfire compartment being disposed at a vertical distance from the topmost lower compartment which is greater than the vertical distance between any given one of the lower compartments and an adjacent lower compartment; and at least one air nozzle for injecting air from the at least one overfire compartment generally in opposition to the rotating fireball along an opposition offset direction which is offset to the other side of the diagonal in a manner such that the injected air promotes the evolution of the rotating fireball into an upward flow; wherein the at least one fuel nozzle of the lowermost fuel injection compartment fires fuel vertically downward and in the opposition offset direction.
 18. The furnace of claim 17 wherein the at least one fuel nozzle of the lowermost fuel injection compartment fires fuel at 15 degrees from the diagonal in the opposition offset direction.
 19. The furnace of claim 17 wherein the lowermost fuel injection compartment includes tilt and yaw controls.
 20. The furnace of claim 19 wherein the lowermost fuel injection compartment is rotatable at ±15° tilt or at ±15° yaw. 